To illustrate this phenomenon, here’s a four-digit number: 5203. Take a metronome, or anything with a steady beat—there are free metronomes for your phone and your computer online—and set it to a beat per second (or you can always go down the old-fashioned route and use a clock that has audible ticks). Now, on every beat, you must add one to each of the numbers and say it out loud; in other words 5-2-0-3 becomes 6-3-1-4, and so on. When you reach the fourth digit, start again from the first – but remember, the number has now changed based on your addition from the round before, so the next four digits you say out loud are 7-4-2-5. It’s very important that you stay on rhythm. Try it for a minute or two. Ready? Go.
Done? You’ve just completed what psychologists call the classic add-one task. How did you do? I’m guessing fairly well – but not quite as well as you’d initially thought. The mandatory rhythm can make it tricky. If it was a piece of cake, try adding two, three, or even four (add-two, add-three, and add-four, respectively) to each of the numbers on each click. The more you add, the harder it gets. In fact, four seems to be about the limit of what even a practiced mind can do without straying from the beat.
Things get even harder when asked to do other tasks. For instance, if you had to stare at a dot on a computer screen at the same time, you would have probably missed any letters or other images that may have flashed on the screen while you were counting. Your eyes would have looked directly at them, yet your brain would have been so preoccupied by counting in a steady pattern that you failed to grasp them entirely. The way our intense focus on a specific thing makes us blind to practically all else is often termed “attentional blindness.” I myself like to call it attentive inattention.
Don’t believe me? Perhaps the most prominent example of this phenomenon is a famous study where a group of students was shown a video of a basketball game and told to count each pass. They did so, to great effect. The only rub? They missed a person in a giant gorilla suit who walked on to the court and deliberately made his way across it, pausing in the middle to thump his chest. They were so busy counting that they failed to see something that seems impossible to miss.
It’s a scary thought, but we are capable of wiping out entire chunks of our visual field without knowingly doing so. Holmes admonished Watson for seeing but not observing. He could have gone a step further: sometimes, we don’t even see.
And it’s far worse than the add-one or even an invisible gorilla would have you believe. We don’t even need to be actively engaged in a cognitively demanding task to let the world pass us by without so much as a realisation of what we’re missing. For instance, when we are in a foul mood, we quite literally see less than when we are happy. Our visual cortex actually takes in less information from the outside world. We could look at the exact same scene twice, once on day that has been going well and once on a day that hasn’t, and we would notice less – and our brains would take in less – on the gloomier of the two days.